JOE ANASTASIO, a slim, dark-haired Web designer for a Wall Street publishing company, was standing outside Madison Square Garden, dressed in black work boots, a torn blue check shirt and a bomber jacket. It was a brisk Sunday morning in the spring, and among the swirl of tourists clutching maps and hockey fans in Rangers jerseys, he might easily have been mistaken for a Metropolitan Transportation Authority track worker heading to a shift.

That is how Mr. Anastasio likes it. A 33-year-old native of Astoria, Queens, he is an urban explorer, to use a term he and his fellow adventurers accept somewhat wearily, along with urban spelunker, infiltrator, hacker and guerilla urbanist. Urban explorers, a highly disparate, loosely knit group, share an obsession with uncovering the hidden city that lies above and below the familiar one all around them. And especially during the summer, they are out in full force.

Alone and with cohorts, Mr. Anastasio has crawled, climbed and sometimes simply brazenly walked into countless train tunnels, abandoned subway stations, rotting factories, storm drains, towers, decaying hospitals and other shadowy remnants of the city’s infrastructure the authorities would rather he did not enter. Although he records his adventures on his Web site, ltvsquad.com, anonymity is, for him, a necessary tool.

A few minutes later on this Sunday morning, Mr. Anastasio was joined by a Korean woman in her 20s named Miru Kim, who with her delicate looks and glossy, shoulder-length black hair offered a striking contrast to Mr. Anastasio’s grizzled appearance. The two headed off, bound for the netherworld beneath their feet.

A few blocks west, they looked around cautiously. Several trucks were parked behind a wire mesh fence, its gate wide open, but no one seemed about. Beyond the fence lay an entrance to the Amtrak tunnels that run north-south along the West Side. They stepped through the gate and headed for the tunnel’s mouth.

Almost immediately, the space became not pitch black, as expected, but a dirty gray, lit by sodium lights and narrow shafts of sunlight from the open street crossings every few blocks above. Faded curlicues of graffiti formed a pattern as dense as wallpaper on the concrete walls.

As the two headed deeper, the sounds of the upper world, of voices and cars, faded. A train thundered past, and the two stepped to one side, averting their faces until its red taillights were dots in the distance. After about 20 minutes, the murky outline of a disused, darker tunnel appeared, and they followed it, holding their flashlights carefully.

This new tunnel ended at a strange contraption, resembling a vast air-conditioner on stilts. Near its base sat the abandoned remains of a homeless person’s encampment: bags of filthy clothes, milk crates full of mismatched sneakers, a few swivel chairs and, lying forlornly in the middle of the tracks, a champagne cork.

Only 20 feet above lay Manhattan’s busy streets, but it might as well have been 20,000 feet, the sense of human desolation was so intense. For Mr. Anastasio, however, the setting was perfect. He whipped out a digital camera and clicked away. A few days later, the photos were up on his Web site. “Don’t you just love this dump?” the text read. “About the only real thing left in NYC is the underground, the dirty, filthy underground.”

Trying to calculate how many urban explorers there are puts one in the hapless position of the reporter who asked Bob Dylan in 1965 how many protest singers there were. “Uh, how many? I think about 136,” Dylan replied sarcastically.

Many American cities have urban exploration Web sites, as do British, Canadian and Australian cities. New York, whose vast infrastructure provides a mecca for those drawn to such things, has dozens of Web sites devoted to recording their owner’s adventures within it.

At the more extreme end are those like Mr. Anastasio’s and nycexposed.com, which is run by a teenager named Sean and contained, until recently, a practical if tongue-in-cheek guide on how to cut through chain-link fences, as well as photographs of speeding subway trains perilously up close.

Not surprisingly, the authorities do not take kindly to such activities.

“Trespassing on the M.T.A.’s infrastructure is not only illegal and extremely dangerous, it’s a pretty stupid idea,” said Jeremy Soffin, a transportation authority spokesman, echoing the sentiments expressed by officials for Amtrak, the New York Police Department and other agencies. “I personally took a track safety class recently, and then you really appreciate how dangerous it is — how big the trains are, how fast-moving they are, and how narrow the spaces are.

“It’s dangerous even for very experienced track workers. There’s no place for urban explorers.”

While Mr. Anastasio and Ms. Kim, a quiet-spoken artist and arts event promoter, have never been arrested while exploring, Mr. Anastasio said he knew some explorers who had been. And many other sites, while they don’t thumb their noses so willfully at authority, are extreme in their own way. Ms. Kim’s site, mirukim.com, which has made her something of a legend in urban explorer circles, contains a section devoted to a project she calls “Naked City Spleen.”

The site features color photographs of Ms. Kim, naked, posed in abandoned tunnels and structures in New York and elsewhere. In one, she crouches like a cat on a vast slab of rusting steel amid the ruins of the former Revere sugar refinery, now demolished, in Red Hook, Brooklyn. In another, she appears, back turned to the camera, squeezed into the narrow heating tunnels below Columbia University, her alma mater. The effect is powerful, not just because of the eroticism, but also because her nakedness seems to emphasize her human vulnerability.

Ms. Kim took considerable risks to obtain her images. A few years ago, she and a friend encountered a body on a trip in Washington Heights. Another time, while she was making a solo visit to the same mysterious tunnel she and Mr. Anastasio visited together, the occupant of the homeless camp appeared just as she had removed her clothes.

Despite her initial fear, she continued with her photography. “In my mind,” she wrote later on her Web site, “he is a dweller in one of the darkest rooms in the collective unconsciousness of all the inhabitants of New York and possibly of all modern cities.”

This sense of communicating with the city on a secret frequency may be what is most appealing to urban explorers.

Steve Duncan is a self-described “guerrilla historian” whose explorations of the city’s forbidden structures — among them the old Croton Aqueduct in the Bronx and the long-closed upper viewing platform 216 feet above the 1964 World’s Fair in Queens — are documented on his Web site, undercity.org.

Photo

A sewer deep beneath Queens.Credit
Steve Duncan

“Most people experience their life in the city in a two-dimensional way,” said Mr. Duncan, a sandy-haired 28-year-old. “You know, they go from Point A to Point B along streets and don’t realize there are these multiple layers to the city. By going 20 feet below or 20 feet above, you can go to a place that is practically unvisited, that maybe 100 people get to see a year.”

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Bent double in their confines one afternoon, sweat dripping from his forehead as the pipes around him wheezed and groaned, he pointed out in a subbasement the remains of the original coal hoppers that fed the boilers before the buildings’ conversion to oil. Beneath another building is part of a 19th-century stone wall that Mr. Duncan said was part of a city insane asylum before being demolished to make way for the university.

Mr. Duncan’s greatest coup came when he wiggled through a vent in the ceiling and emerged from a door on the other side of a room. A quick step through the door and across the corridor outside led to a densely cluttered room, piled high with cases of ancient electrical machinery.

This, Mr. Duncan announced, was the original Pupin Laboratory, where the university’s physics department built a particle accelerator and split the atom in 1939, in an early stage of what would be known as the Manhattan Project. Mr. Duncan said he believed that in 1997 he became the first urban explorer to discover it, although others followed suit, as attested by the graffiti around the room.

The particle accelerator — a circular green mass in the center of the room that resembles nothing more alarming than an enormous food processor — was too heavy and too dangerous to safely remove after the project moved to Chicago, following the attack on Pearl Harbor, he said, so the university decided to keep it here, “in their mildly radioactive junk storage room.”

The discovery left him jubilant.

“It’s just a great example of how you peel back one layer and you get to old coal hoppers,” he said. “You peel back another layer and you find the foundations of an asylum when this area was all grass and farmlands. You peel back another layer, and here’s the building where the atom was split.”

For some urban explorers, the search for shadow cities does not entail venturing down tunnels or scaling high walls. Kevin Walsh, the 50-year-old, Brooklyn-born creator of the Web site forgotten-ny.com — a vast cornucopia of facts, photographs, conjecture, mythology and infrastructure — rarely goes urban exploring in the guerrilla sense of the term.

Instead, armed with a camera and the combined knowledge of a small library of books on New York, he stalks the city’s streets looking for its secrets hidden in plain view. From faded advertisements to ancient streetlights to streets named after long-obscure luminaries, he obsessively records the ephemera of what he terms “the lost metropolis” on his Web site. Much of this information is collected in his book, “Forgotten New York,” which was published last year and is grist for the tours he conducts of forgotten corners of the city.

During a recent stroll with Mr. Walsh around Green-Wood Cemetery in Brooklyn, it became clear that his love of the city’s ephemera goes beyond brick and stone. While on a hunt for the gravestone of the infamous 19th-century figure Bill the Butcher, he noticed some ancient lovers’ graffiti carved into a tree trunk near the gravestone.

“That’s what I love!” he said as he examined the blend of hearts and names, their edges softened and indecipherable with age. “That’s what I show people on trips.”

Beyond the thrill of seeing what others have not seen, or dare not see, and the sense that it should be recorded for future generations, urban explorers are driven by another motive. It is impossible to visit some of their more spectacular haunts without experiencing a touch of the sacred.

This was apparent one afternoon when Mr. Duncan’s good friend and co-conspirator on numerous adventures, Moses Gates, a 31-year-old tour guide and graduate student in urban planning, undertook a journey into the abandoned Red Hook Grain Terminal on Brooklyn’s waterfront.

“Generally, climbing urban structures and being high up really allows me to connect with the city,” Mr. Gates said, “although I sometimes get that connection from other places, or just from walking around town. I love the feeling of being at one with the city — it’s a spiritual experience, I won’t deny it.”

The grain terminal is one of the waterfront’s industrial masterpieces, a series of 54 concrete silos about 12 stories high, built in 1922 to hold grain arriving by barge from the West. The cold gray waters of the Erie Basin lapping around the structure’s edges give it the sense of an island fortress.

The terminal was decommissioned in the 1960s and now stands in a small industrial park, surrounded by concrete walls. Recently, a 17-year-old plan to turn it into a recycling center was revived, though its future remains uncertain. Mr. Gates negotiated the walls, then swung himself lithely beneath a rusted steel grating at one corner of the building.

Suddenly he was inside what might at first glance have been mistaken for a cathedral. Fat concrete columns lined up as far as the eye could see, creating a dreamlike procession of naves in all directions. Light filtered in from the sides, casting long diagonal shadows across the floor.

But what really gave the building its rarefied air was the silence. Amid the daily cacophony of the city, where every place is packed with a scrum of people, this space stood empty, a still counterpoint to everything around it.

Mr. Gates began to climb the corroded metal stairs that led to the roof. Graffiti lined the inner walls — a good sign. “Graffiti artists are almost always first,” Mr. Gates said. “If there’s no graffiti, there’s a good chance it’s impossible to get there.”

At the end of his climb, as he popped his head out of a hatch on the roof, a magnificent — and utterly illicit — 360-degree view of the city opened up. In the foreground lay Red Hook’s 19th-century industrial sprawl of warehouses and narrow streets lined with row houses. In the distance rose Manhattan’s dull gray skyline. Tiny cars crawled along the elevated Prospect Expressway, an F train made its way over the Gowanus Canal, and airplanes banked steeply as they headed for Kennedy Airport.

“Planes, trains and automobiles, you got it all here,” Mr. Gates said happily. Pausing to look out at this perspective, seen by so few, he added: “There’s no doubt about it. You’ve got romance here.”

Correction: August 5, 2007

An article last Sunday about people who explore the city’s abandoned places misstated the year when one such person, Steve Duncan, says he discovered a former laboratory used in an early stage of the Manhattan Project at Columbia University. It was 1997, not 1987.

Ben Gibberd’s book “New York Waters: Profiles From the Edge,” with the photographer Randy Duchaine, was published in May by Globe Pequot Press.